This invention relates to a method of manufacturing a domestic moulded-on mains plug of the type comprising an electrically-insulating base provided with conductive contact members, such as metal pins which project from one side of the base for insertion into an electrical socket, and which are connected to respective wires of an electrical cable by connections on the other side of the base. The plug includes an electrically-insulating cap which extends over at least two of the wire connections, and an injection-moulded plug body moulded onto the base and enclosing the cap, the connections, and the end of the cable.
A plug of the type defined is known from British patent specifications Nos. 1,581,661 and 1,595,782. In each of these plugs the insulating cap is provided over at least the live, or high voltage, connections as a safety measure to ensure that if any strands of wire are displaced during the injection moulding of the plug body onto the base, these strands cannot reach the surface of the plug body where they could be touched by the user. In order to ensure that the insulating cap is incorporated in each plug, each of the abovementioned patent specifications discloses an arrangement in which a portion of the insulating cap is visible on the outer surface of the finished plug.
Visual inspection methods, however, are generally unsatisfactory in that their absolute effectiveness cannot be guaranteed because they require the full continuous concentration of the operators concerned. Fatigue, eye strain, or momentary distraction of the operator(s) could, in the present case, allow plugs without caps to escape detection, with possibly highly dangerous results.
Another disadvantage of visual inspection methods is that positive action must be taken to destroy any plugs that fail the visual inspection test. In that fairly close inspection is required to identify faulty plugs, such plugs must be rendered unusable at the time they are inspected visually. Otherwise the lack of immediately-obvious fault indications could result in dangerous plugs being released for use.
British patent specification No. 2,088,654 discloses a fused plug which is automatically rendered unusable if the insulating cap is not during in place in the moulding-on process. The plug concerned has a recess between the pins to accommodate the fuse and there is at least one hole through the base into the recess. If a cap is provided, the cap closes this hole. If no cap is provided, however, the plastic material which is injected during the moulding stage flows through the hole into the fuse recess. Even if a faulty plug is not detected at the end of the moulding-on process, it will be inevitably rejected at the next stage during which the fuse has to be fitted, since it is not possible to fit the fuse in the recess. This not only eliminates the need for the visual inspection, but also automatically renders the plug unusable.
In all the plugs disclosed in the above-mentioned specifications, the lack of an insulating cap is not detected until after the moulding-on stage--i.e. when the whole manufacturing process has been completed except for fitting the fuse. Thus in each case the material and labour costs of a fully manufactured plug, with its attached cable, are wasted in the event of a missing cap.